Allogeneic blood or marrow transplantation, or alloBMT, is a potentially curative therapy for a wide variety of chemotherapy-incurable hematologic malignancies. However, the overall success of alloBMT remains constrained by procedure-related toxicity, which limits eligibility to relatively young patients with histocompatible donors, and by lack of efficacy, manifested as post-transplantation progression of the underlying disease. In the previous funding period, we had conducted clinical trials of post-transplantation, high dose cyclophosphamide (Cy) to reduce toxicity by promoting bi-directional transplantation tolerance while sparing stem cells and preserving immune responses to infection. A major goal of Aim 1 of the current proposal is to characterize in detail the mechanisms of Cy-induced tolerance and the effect of post- transplantation Cy on regulatory T cells, antigen-presenting cells, and antigen-specific T cells. To increase the anti-tumor efficacy of allogeneic T cell infusions, we will focus on inducing tumor-specific immunity by two distinct approaches. The first approach is to administer cell-based tumor vaccines, with or without donor lymphocyte infusion, after nonmyeloablative allogeneic stem cell transplantation (Aim 2). The second approach will be to treat non-BMT candidates with Cy followed by transiently engrafting allogeneic lymphocytes so as to induce an "allogeneic effect" that breaks tolerance in the patient's tumor-specific T cells (Aim 3). We will also study the capacity of tumor-targeted therapies to augment the effect of administering allogeneic lymphocytes in this context (Aim 4). The ultimate goals of this project are to establish post- transplantation Cy as the gold standard of GVHD prophylaxis and to extend the application of adoptive cellular immunotherapy with allogeneic T cells outside of the context of alloBMT. Cancer is now the leading killer of Americans under the age of 85. Transfusions of white blood cells from a healthy donor can actually make a cancer shrink, but this is a dangerous treatment that doesn't always work. The goal of this project is to find ways to make white blood cell transfusions safer and more effective in the treatment of cancer.